Catalytic reforming is a refinery process used to improve octane quality of liquid hydrocarbons such as naphthas or straight run gasolines. Reforming can be considered as changing the molecular structure of various hydrocarbons in a hydrocarbon feedstock to produce a reformed product. Such change is generally carried out by combinations of chemical reactions involving dehydrogenation, dehydrocyclization, isomerization, and hydrocracking of the various hydrocarbons. The reformed product is typically referred to as reformate.
The reforming process is carried out using a reforming catalyst, which becomes coked as the process is carried out. A multifunctional catalyst is usually employed, which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component or components.
Reforming reactions are both endothermic and exothermic. Endothermic reactions are generally predominant, particularly in the early stages of reforming.
Numerous reactor arrangements have been designed to accommodate the complex of endothermic and exothermic reactions that take place. These arrangements typically fall into three major categories: semi-regenerative, cyclic, and continuous. Fixed bed reactors are typically employed in semi-regenerative and cyclic reforming. Moving bed reactors are typically employed in continuous reforming.
In semi-regenerative reforming, the entire reforming process unit is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by coke deposition. As catalyst activity becomes undesirably reduced due to the coke deposition, the entire unit is shut down for regeneration and reactivation of the catalyst.
In cyclic reforming, multiple reactors are in series and the reactors are individually isolated such that catalytic reforming or catalyst regeneration can be carried out as desired in any individual reactor. In effect, one or more reactors can be swung out of line by various piping arrangements to carry out the reforming and regeneration reactions at the same time or in unison. The “swing reactor,” in essence, temporarily replaces an on-line reactor, so that the coked catalyst can be regenerated in the swing reactor, while reforming of the hydrocarbon feed continues in the on-line reactor. Once regeneration is complete, the swing reactor can then be rotated back into service.
In continuous reforming, the reactors are typically moving bed reactors. In a moving bed reactor, reforming catalyst is continuously added and withdrawn. The catalyst that is withdrawn is regenerated in a separate vessel and then returned to the moving bed reactor.
A variety of process variables have been utilized to improve reformate product quality, such as C5+ liquid yield and/or octane quality. For example, if a product of high octane is desired, e.g., 100 or higher RON (research octane number), it has been typical to obtain such a product by increasing space velocity and/or reaction temperature. However, changing these variables in such a way often results in reducing the desired yield of C5+ components in the product, as well as in decreasing the activity of the catalyst at a faster rate due to enhanced coke formation on the catalyst during the reaction process.
U.S. Pat. No. 5,203,966 discloses a process for catalytically reforming a gasoline boiling range feedstock described as resulting in a significantly higher yield of C5+ liquid product, as well as hydrogen, as a percent of the naphtha feedstock. The reforming process is carried out in multiple stages, with an aromatics-rich (high octane) stream being separated between stages. The separation is performed after reforming at low severity, in a first stage or set of stages, to convert most of the alkylcyclohexanes and alkylcyclopentantes to aromatics with minimum cracking of paraffins.
U.S. Pat. No. 3,716,477 discloses a reforming process in which a naphtha feed is reformed to produce a high octane gasoline product. The process is carried out at a pressure from 20-100 psig (140-690 kPag), a hydrogen or hydrocarbon mole ratio of less than 2, and a liquid hourly space velocity of 0.5-5, using a catalyst comprising platinum and rhenium supported on a porous inorganic oxide carrier. The reforming process is periodically discontinued to permit regeneration of the catalyst to restore substantially its initial activity.
Nevertheless, there is still much room for improving the reforming process. In particular, higher octane reformate is desired with increased demand for yield. Achieving higher yields of such product, while being able to more adequately control coke build-up and producing increased quantities of hydrogen as an additional product, is of particular demand. Sources of additional hydrogen are also in demand, to enable refineries to produce higher yields of higher quality fuels lower in sulfur components.